Mistletoe extracts have been therapeutically used for centuries. Since the beginning of this century, mistletoe preparations have been used in cancer therapy with varying success [Bocci, 1993; Gabius et al., 1993; Gabius & Gabius, 1994; Ganguly & Das, 1994]. Hajto et al. [1989, 1990] could show that the therapeutic effects are mediated in particular by socalled mistletoe lectins (viscumins, Viscum album agglutinins, VAA). Besides a cytotoxic effect today the art in particular discusses (unspecific) immunostimulation, the positive effects of which are used for the accompanying therapy and after-care of tumor patients. An increase in quality of life is possibly mediated in such patients by the secretion of endogeneous endorphins [Heiny and Beuth, 1994]. Numerous in vitro [Hajto et al., 1990; Mannel et al., 1991; Beuth et al., 1993] and in vivo [Hajto, 1986; Hajto et al., 1989; Beuth et al., 1991; Beuth et al., 1992] studies as well as clinical studies [Beuth et al., 1992] report an increased release of inflammatory cytokines (TNF-.alpha., IL-1, IL-6) as well as an activation of cellular components of the immunological system (T.sub.H cells, NK cells).
Today a 60 kDa mistletoe lectin protein is considered the active principle of the mistletoe extracts which can be biochemically obtained from extracts [Franz et al., 1977; Gabius et al., 1992]. The ML protein consists of two covalently S--S linked subunits, the A chain of which being responsible for the enzymatic inactivation of ribosomes [Endo et al., 1988] and its B chain being responsible for carbohydrate binding. Biological activity today is mainly attributed to the lectin activity of the B chain [Hajto et al., 1990].
However, little is known about the structure/function relationships of the mistletoe lectin (ML). The contribution which the single-chains and their different biochemical and enzymatic activities make to the mode of action and/or the therapeutic effects observed is not yet clear. The analysis of the structure/function relationships is rendered difficult by contamination of the preparations with other plant ingredients of the mistletoe [Stein & Berg, 1994]. It is discussed that the activity of extract preparations may depend on the different compositions of the preparations, which in turn may depend on the kind of the host tree (e.g., apple, pine, poplar) [Hulsen et al., 1986]. Both viscotoxins [Garcia-Olmedo et al., 1983; Mendez et al., 1990] and other viscumins (such as ML-2, ML-3) are said to have similar effects [Eifler et al., 1994]. Even ML preparations which have been highly purified biochemically (affinity chromatography) are substantially heterogeneous (FIG. 8).
This heterogeneity relates to the biochemically assessable activities of the chains, to the evoked in vitro and in vivo effects and to the protein structures as such. Structural variants are discussed for the glycosylations of the ML A and B chains as well as for sequence variations. Gabius et al. [1992] and Dietrich et al. [1992] show a sequence variability of the A1 and A2 chains of ML-1.
For a more detailed analysis of the therapeutic effects of the mistletoe lectin it is desirable that it is available as structurally homogeneous substance in pure form. Furthermore, it is important for the scientists to be able to prepare mistletoe lectin or its components in large amounts in pure form so that it/they can be employed on a large scale as active ingredient of pharmaceutical compositions. These aims could not be nearly attained by the processes so far known in the art. Isolation from plant material according to the present state of the art will always yield a heterogeneous mixture of substances. The heterogeneity of plant mistletoe lectin preparations inter alia results from the posttranslational processing of the ML-1 to the isoforms ML-2 and ML-3 so that mistletoe lectin preparations have a varying content of ML-1, ML-2 and ML-3 depending on the method of isolation or the duration of fermentation [Jaggy et al., 1995]. Any of the above-mentioned isoforms additionally is largely microheterogeneous which is illustrated in FIG. 8 for ML-1 by isoelectric chromatofocusing.